Be careful, however.If you think the cat is really allergic to the thing and he's showing another signs or throwing up many times , please contact a doctor
I dont know about an alternative to amoxicillin but maybe you could give your cat a gastric protector previously to that.
One of my cats had the same problem once with sulpha _ now I not only avoid liquid medicine but also got the vet to agree in me using those pills about half hour before.
Here in my country we use one for humanes,( OMEPRAZOL)its not expensive but you better ask a vet for the right dosage